14 CFR § 125.205 - Equipment requirements: Airplanes under IFR.
(a) A vertical speed indicator;
(b) A free-air temperature indicator;
(c) A heated pitot tube for each airspeed indicator;
(d) A power failure warning device or vacuum indicator to show the power available for gyroscopic instruments from each power source;
(e) An alternate source of static pressure for the altimeter and the airspeed and vertical speed indicators;
(f) At least two generators each of which is on a separate engine, or which any combination of one-half of the total number are rated sufficiently to supply the electrical loads of all required instruments and equipment necessary for safe emergency operation of the airplane; and
(g) Two independent sources of energy (with means of selecting either), of which at least one is an engine-driven pump or generator, each of which is able to drive all gyroscopic instruments and installed so that failure of one instrument or source does not interfere with the energy supply to the remaining instruments or the other energy source. For the purposes of this paragraph, each engine-driven source of energy must be on a different engine.
(h) For the purposes of paragraph (f) of this section, a continuous inflight electrical load includes one that draws current continuously during flight, such as radio equipment, electrically driven instruments, and lights, but does not include occasional intermittent loads.
(i) An airspeed indicating system with heated pitot tube or equivalent means for preventing malfunctioning due to icing.
(j) A sensitive altimeter.
(k) Instrument lights providing enough light to make each required instrument, switch, or similar instrument easily readable and installed so that the direct rays are shielded from the flight crewmembers' eyes and that no objectionable reflections are visible to them. There must be a means of controlling the intensity of illumination unless it is shown that nondimming instrument lights are satisfactory.